+12 constant to momentary

First off I am trying to reset a device when the car is started by pulsing it's power line for half a second or so.. (I have a +12v line that's active when the car is running) I have a 12V relay, a 1000 uf Cap and some resistors. I am testing with a bench supply right now so I don't know if that's part of my problem, but I will try it on my battery later tonight.

I have the + power for my device connected to the NC lines on the relay so when it's actuated the power is off otherwise it's powered up.

I have the - side of the 12v supply connected to the end of the cap which is in turn connected to the trigger lines on the relay. the other end has +12v on it. The first time I connect the +12v it toggles and works perfectly but then after that it doesn't work again until I manually short out the cap to discharge it.

I tried putting a resistor and a light across the cap to discharge it but when that is in place the relay latches and does not give momentary pulses anymore.

I am at a loss at this point and hoping someone can tell me what I am doing wrong.

Thanks in advance guys.

So you are using....

(from Relay Diagrams - Constant to Momentary Output - Negative Input/Positive Output)

Yes.

That's the circuit I tried to create.

Without the resistor I have to drain the cap manually and with the resistor the relay latches.

Just tried it in my car with the battery and it has the exact same results.

Latches with resistor and only toggles until the cap charges then it won't toggle anymore until it's discharged manually.

I know that circuit works flawlessly.  Did you put the Diode across 85 and 86?. 

Yes I did, I tried with and without.

Maybe I am wiring it wrong somehow?

The relay has 2 pins for the coils

One pin has the positive side of the cap on it then attached between the legs of the cap is the resistor on the negative end of the cap I have the (-) terminal.

The other coil lead has just the (+) lead on it.

There is a diode running from one pin to the other.

I am new at this, is this wired the way it should be or did I do something wrong?

Thanks.

The resistor value is too low (else the coil too high).

It is in series with the coil, hence a resistor divider.

But 10k should be fine.

Maybe if you are using 1k or less.
IE - if it's a 250R (Ohm) coil, then 250T-1000R = 1R:4R is a 1:5 divider (from 250/(250+1k) or 1/(1+4) = 1/5).

So at say 12V, 12/5 = 2.4V across the coil. Probably not enough to pull it in (ie, switch on or energise the relay), but likely to hold in - ie, it won't let go.


Increasing the resistor value merely increases the time the relay stays on - ie, it takes the cap longer to drain (through a larger resistor).


The diode isn't necessary though its inclusion means instant cap discharge if the "fused +12V" is grounded when NOT present. EG - an IGN switch is +12V when on, but MAY be effectively grounded when off because of other things it powers (ie, any loads IGN powers are grounded; even if only relays or meters etc, they can form a reasonable ground path.)


FYI - I now question how many circuits I have stated that the diode is "only" for spike suppression when it may have some other function. (What was it the other day....?)
Hence I rarely include spike suppression diodes - IMO they should be a "general caveat" for any relay, though I might add a "don't forget..." or "suppression not included", else include it with "for suppression only". I can't recall where anyone has stated "diode is required for operation...".

Thanks.
I don't care so much about how long it stays on or off so much but I need it to toggle every time it gets power.

I don't have a lot of resistors here and I don't know if I have a 10k or not.. I will poke them with the multimeter and see what I get.

If the resistance is too low it will stay on? I will get some higher value resistors and try again if you think that will make a difference.

Here is the relay specs:

12VDC 4PDT relay
75MA 160 ohm coil
5A@28vdc or 250 vac

Thanks.

I got a 10 k trim pot because all my resistors were like 50 ohms and it worked. Guess following directions helps :P

Thanks for the support.